Mobile electronic devices such as cell-phone, portable audio player and like are widely used currently. Lithium-ion secondary batteries are widely used for such mobile electronic devices.
When function of the mobile electronic devices increases, power consumption of the mobile electronic devices increases too, and therefore it is desired to prolong service life of the batteries.
To meet such a requirement of the market, a new type lithium-ion secondary battery is brought to the market (see Non-Patent Document 1), which is developed based on a conventional lithium-ion secondary battery. Compared to the conventional lithium-ion battery, the current capacity of the new type battery is increased by 30%.
FIG. 21 is a graph showing the discharging characteristic of the new type battery. The new type battery has large discharged capacity and long service life compared with the conventional battery. Further, the discharge voltage possible to be stably outputted by the new type battery is expanded to around 2V.
Although the conventional lithium-ion battery has high discharge voltage, since the voltage drops rapidly as the discharge voltage becomes 3V or lower (see E1 of FIG. 21), the discharge cutoff voltage is set to 3V so as to prevent over discharge. In such a case, since the output voltage is 2.5V, the battery only needs to be provided with a DC/DC converter having a power circuit which performs step-down control only.
In contrast, in order to make better use of the new type lithium-ion secondary battery, an input voltage down to around 2.0V (which is lower than the output voltage) also needs to be coped with. In other words, in addition to the step-down control, the DC/DC converter also needs to perform step-up control, and further, it is necessary to continuously and smoothly perform switching control between the step-down control and the step-up control so as to output a voltage of 2.5V (which is the output voltage corresponding to the load using the conventional battery) can be outputted.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2002-233139    [Patent Document 2] Japanese published examined application No. 58-40913.    [Non-Patent Document 1] “Changing negative-electrode increases capacity by 30%” - - - Sony's new type lithium-ion secondary battery”, Nikkei Electronics, p. 34-35, Feb. 28, 2005    [Non-Patent Document 2] “About Characteristic of DC-DC Converter Controlled by Delta-Sigma Modulation”, IMAMURA Yasuhide, TANAKA Tetsurou YOSHIDA Hiroshi, Technical report of IEICE, EE2002-78